Download Delphi Yacc & Lex A parser generator toolset for Delphi and

Delphi Yacc & Lex
A parser generator toolset for Delphi and Kylix
Version 1.4 User Manual
Michiel Rook
[email protected]
December 2005
Contents
1 Introduction
4
1.1 Original introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4
1.2 Original credits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5
1.3 License . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5
2 Getting Started
6
3 Delphi Yacc
7
3.1 Usage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7
3.2 Options . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7
3.3 Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7
3.4 Yacc Source . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7
3.5 Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8
3.6 Grammar Rules and Actions
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
9
3.7 Auxiliary Procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
12
3.8 Lexical Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
12
3.9 How The Parser Works . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
13
3.10 Ambigious Grammars . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
17
3.11 Error Handling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
22
3.12 Yacc Library . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
23
3.13 Other Features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
23
3.14 Implementation Restrictions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
23
3.15 Differences from UNIX Yacc . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
24
4 Delphi Lex
25
4.1 Usage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
25
4.2 Options . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
25
4.3 Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
25
4.4 Lex Source . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
26
4.5 Regular Expressions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
27
4.6 Start Conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
28
4.7 Lex Library . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
29
2
4.8 Implementation Restrictions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
29
4.9 Differences from UNIX Lex . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
30
3
1
1
INTRODUCTION
Introduction
This document is the user manual for Delphi Yacc & Lex, a parser generator toolset for Delphi
and Kylix. Delphi Yacc & Lex is based on Turbo Pascal Lex and Yacc, version 4.1, by Albert Graef
et al. The primary goal is to clean up the code, and improve compatibility and maintainability with
modern versions and compilers of the Pascal language.
This manual is a direct adaptation of the original manual of TP Lex and Yacc Version 4.1a, and
the original introduction and credits are preserved below.
1.1
Original introduction
This document describes the TP Lex and Yacc compiler generator toolset. These tools are designed especially to help you prepare compilers and similar programs like text processing utilities
and command language interpreters with the Turbo Pascal (TM) programming language.
TP Lex and Yacc are Turbo Pascal adaptions of the well-known UNIX (TM) utilities Lex and Yacc,
which were written by M.E. Lesk and S.C. Johnson at Bell Laboratories, and are used with the
C programming language. TP Lex and Yacc are intended to be approximately “compatible” with
these programs. However, they are an independent development of the author, based on the
techniques described in the famous “dragon book” of Aho, Sethi and Ullman (Aho, Sethi, Ullman:
Compilers : principles, techniques and tools, Reading (Mass.), Addison-Wesley, 1986).
Version 4.1 of TP Lex and Yacc works with all recent flavours of Turbo/Borland Pascal, including
Delphi, and with the Free Pascal Compiler, a free Turbo Pascal-compatible compiler which currently runs on DOS and Linux (other ports are under development). Recent information about TP
Lex/Yacc, and the sources are available from the TPLY homepage:
http://www.musikwissenschaft.uni-mainz.de/~ag/tply
For information about the Free Pascal Compiler, please refer to:
http://www.freepascal.org
TP Lex and Yacc, like any other tools of this kind, are not intended for novices or casual programmers; they require extensive programming experience as well as a thorough understanding
of the principles of parser design and implementation to be put to work successfully. But if you
are a seasoned Turbo Pascal programmer with some background in compiler design and formal
language theory, you will almost certainly find TP Lex and Yacc to be a powerful extension of your
Turbo Pascal toolset.
This manual tells you how to get started with the TP Lex and Yacc programs and provides a short
description of these programs. Some knowledge about the C versions of Lex and Yacc will be
useful, although not strictly necessary. For further reading, you may also refer to:
• Aho, Sethi and Ullman: Compilers : principles, techniques and tools. Reading (Mass.),
Addison-Wesley, 1986.
• Johnson, S.C.: Yacc – yet another compiler-compiler. CSTR-32, Bell Telephone Laboratories, 1974.
4
1.2
Original credits
1 INTRODUCTION
• Lesk, M.E.: Lex – a lexical analyser generator. CSTR-39, Bell Telephone Laboratories,
1975.
• Schreiner, Friedman: Introduction to compiler construction with UNIX. Prentice-Hall, 1985.
• The Unix Programmer’s Manual, Sections ‘Lex’ and ‘Yacc’.
1.2
Original credits
I would like to thank Berend de Boer ([email protected]), who adapted TP Lex and Yacc to
take advantage of the large memory models in Borland Pascal 7.0 and Delphi, and Michael Van
Canneyt ([email protected]), the maintainer of the Linux version of the
Free Pascal compiler, who is responsible for the Free Pascal port. And of course thanks are due
to the many TP Lex/Yacc users all over the world for their support and comments which helped
to improve these programs.
1.3
License
This program is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; either version 2 of the License, or
(at your option) any later version.
This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU General Public License for more details.
You should have received a copy of the GNU General Public License
along with this program; if not, write to the Free Software
Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307
5
USA
2
2
GETTING STARTED
Getting Started
Instructions on how to compile and install Delphi Lex and Yacc on all supported platforms can be
found in the README file contained in the distribution.
Once you have installed Delphi Lex and Yacc on your system, you can compile your first Delphi
Lex and Yacc program expr. Expr is a simple desktop calculator program contained in the distribution, which consists of a lexical analyzer in the Delphi Lex source file exprlex.l and the parser
and main program in the Delphi Yacc source file expr.y. To compile these programs, issue the
commands
dlex exprlex
dyacc expr
That’s it! You now have the sources (exprlex.pas and expr.pas) for the expr program. Use the
Delphi compiler to compile these programs as follows:
dcc32 expr
(Of course, the precise compilation command depends on the type of compiler you are using.
Thus you may have to replace dcc32 with dcc, depending on the version of the Delphi compiler
you have.)
Having compiled expr.pas, you can execute the expr program and type some expressions to
see it work (terminate the program with an empty line).
The Delphi Lex and Yacc programs recognize some options which may be specified anywhere on
the command line. E.g.,
dlex -o exprlex
runs Delphi Lex with “DFA optimization” and
dyacc -v expr
runs Delphi Yacc in “verbose” mode (Delphi Yacc generates a readable description of the generated parser).
The Delphi Lex and Yacc programs use the following default filename extensions:
• .l: Delphi Lex input files
• .y: Delphi Yacc input files
• .pas: Delphi Lex and Yacc output files
As usual, you may overwrite default filename extensions by explicitly specifying suffixes.
If you ever forget how to run Delphi Lex and Yacc, you can issue the command ldex or dyacc
without arguments to get a short summary of the command line syntax.
6
3
3
DELPHI YACC
Delphi Yacc
This section describes the Delphi Yacc compiler compiler.
3.1
Usage
dyacc [options] yacc-file[.y] [output-file[.pas]]
3.2
Options
"-v" “Verbose:” Delphi Yacc generates a readable description of the generated parser, written
to yacc-file with new extension .lst.
"-d" “Debug:” Delphi Yacc generates parser with debugging output.
3.3
Description
Delphi Yacc is a program that lets you prepare parsers from the description of input languages by
BNF-like grammars. You simply specify the grammar for your target language, augmented with
the Delphi code necessary to process the syntactic constructs, and Delphi Yacc translates your
grammar into the Delphi code for a corresponding parser subroutine named yyparse.
Delphi Yacc parses the source grammar contained in yacc-file (with default suffix .y) and writes
the constructed parser subroutine to the specified output-file (with default suffix .pas); if no
output file is specified, output goes to yacc-file with new suffix .pas. If any errors are found
during compilation, error messages are written to the list file (yacc-file with new suffix .lst).
The generated parser routine, yyparse, is declared as:
function yyparse : Integer;
This routine may be called by your main program to execute the parser. The return value of the
yyparse routine denotes success or failure of the parser (possible return values: 0 = success, 1
= unrecoverable syntax error or parse stack overflow).
Similar to Delphi Lex, the code template for the yyparse routine may be found in the yyparse.cod
file. The rules for locating this file are analogous to those of Delphi Lex (see Section Delphi Lex).
The Delphi Yacc library (YaccLib) unit is required by programs using Yacc- generated parsers;
you will therefore have to put an appropriate uses clause into your program or unit that contains
the parser routine. The YaccLib unit also provides some routines which may be used to control
the actions of the parser. See the file yacclib.pas for further information.
3.4
Yacc Source
A Delphi Yacc program consists of three sections separated with the %% delimiter:
7
3.5
Definitions
3
DELPHI YACC
definitions
%%
rules
%%
auxiliary procedures
The Delphi Yacc language is free-format: whitespace (blanks, tabs and newlines) is ignored, except if it serves as a delimiter. Comments have the C-like format /* ... */. They are treated
as whitespace. Grammar symbols are denoted by identifiers which have the usual form (letter, including underscore, followed by a sequence of letters and digits; upper- and lowercase is
distinct). The Delphi Yacc language also has some keywords which always start with the % character. Literals are denoted by characters enclosed in single quotes. The usual C-like escapes
are recognized:
• \n denotes newline
• \r denotes carriage return
• \t denotes tab
• \b denotes backspace
• \f denotes form feed
• \nnn denotes character no. nnn in octal base
3.5
Definitions
The first section of a Delphi Yacc grammar serves to define the symbols used in the grammar. It
may contain the following types of definitions:
• start symbol definition: A definition of the form
%start symbol
declares the start nonterminal of the grammar (if this definition is omitted, Delphi Yacc
assumes the left-hand side nonterminal of the first grammar rule as the start symbol of the
grammar).
• terminal definitions: Definitions of the form
%token symbol ...
are used to declare the terminal symbols (“tokens”) of the target language. Any identifier
not introduced in a %token definition will be treated as a nonterminal symbol.
As far as Delphi Yacc is concerned, tokens are atomic symbols which do not have an innert
structure. A lexical analyzer must be provided which takes on the task of tokenizing the
input stream and return the individual tokens and literals to the parser (see Section Lexical
Analysis).
• precedence definitions: Operator symbols (terminals) may be associated with a precedence
by means of a precedence definition which may have one of the following forms
8
3.6
Grammar Rules and Actions
3 DELPHI YACC
%left symbol ...
%right symbol ...
%nonassoc symbol ...
which are used to declare left-, right- and nonassociative operators, respectively. Each
precedence definition introduces a new precedence level, lowest precedence first. E.g.,
you may write:
%nonassoc ’<’ ’>’ ’=’ GEQ LEQ NEQ
/* relational operators */
%left
’+’ ’-’ OR
/* addition operators */
%left
’*’ ’/’ AND
/* multiplication operators */
%right
NOT UMINUS
/* unary operators */
A terminal identifier introduced in a precedence definition may, but need not, appear in a
%token definition as well.
• type definitions: Any (terminal or nonterminal) grammar symbol may be associated with a
type identifier which is used in the processing of semantic values. Type tags of the form
<name> may be used in token and precedence definitions to declare the type of a terminal
symbol, e.g.:
%token <Real> NUM
%left <AddOp> ’+’ ’-’
To declare the type of a nonterminal symbol, use a type definition of the form:
%type <name> symbol ...
e.g.:
%type <Real> expr
In a %type definition, you may also omit the nonterminals, i.e. you may write:
%type <name>
This is useful when a given type is only used with type casts (see Section Grammar Rules
and Actions), and is not associated with a specific nonterminal.
• Delphi declarations: You may also include arbitrary Delphi code in the definitions section,
enclosed in %{ %}. This code will be inserted as global declarations into the output file,
unchanged.
3.6
Grammar Rules and Actions
The second part of a Delphi Yacc grammar contains the grammar rules for the target language.
Grammar rules have the format
9
3.6
Grammar Rules and Actions
3 DELPHI YACC
name : symbol ... ;
The left-hand side of a rule must be an identifier (which denotes a nonterminal symbol). The righthand side may be an arbitrary (possibly empty) sequence of nonterminal and terminal symbols
(including literals enclosed in single quotes). The terminating semicolon may also be omitted. Different rules for the same left-hand side symbols may be written using the | character to separate
the different alternatives:
name : symbol ...
| symbol ...
...
;
For instance, to specify a simple grammar for arithmetic expressions, you may write:
%left ’+’ ’-’
%left ’*’ ’/’
%token NUM
%%
expr : expr ’+’
| expr ’-’
| expr ’*’
| expr ’/’
| ’(’ expr
| NUM
;
expr
expr
expr
expr
’)’
(The %left definitions at the beginning of the grammar are needed to specify the precedence and
associativity of the operator symbols. This will be discussed in more detail in Section Ambigious
Grammars.)
Grammar rules may contain actions – Delphi statements enclosed in { } – to be executed as the
corresponding rules are recognized. Furthermore, rules may return values, and access values
returned by other rules. These “semantic” values are written as $$ (value of the left-hand side
nonterminal) and $i (value of the ith right-hand side symbol). They are kept on a special value
stack which is maintained automatically by the parser.
Values associated with terminal symbols must be set by the lexical analyzer (more about this in
Section Lexical Analysis). Actions of the form $$ := $1 can frequently be omitted, since it is the
default action assumed by Delphi Yacc for any rule that does not have an explicit action.
By default, the semantic value type provided by Yacc is Integer. You can also put a declaration
like
%{
type YYSType = Real;
%}
into the definitions section of your Yacc grammar to change the default value type. However, if
you have different value types, the preferred method is to use type definitions as discussed in
Section Definitions. When such type definitions are given, Delphi Yacc handles all the necessary
10
3.6
Grammar Rules and Actions
3 DELPHI YACC
details of the YYSType definition and also provides a fair amount of type checking which makes it
easier to find type errors in the grammar.
For instance, we may declare the symbols NUM and expr in the example above to be of type Real,
and then use these values to evaluate an expression as it is parsed.
%left ’+’ ’-’
%left ’*’ ’/’
%token <Real> NUM
%type <Real> expr
%%
expr : expr ’+’ expr
| expr ’-’ expr
| expr ’*’ expr
| expr ’/’ expr
| ’(’ expr ’)’
| NUM
;
{
{
{
{
{
$$
$$
$$
$$
$$
:=
:=
:=
:=
:=
$1+$3;
$1-$3;
$1*$3;
$1/$3;
$2;
}
}
}
}
}
(Note that we omitted the action of the last rule. The “copy action” $$ := $1 required by this rule
is automatically added by Delphi Yacc.)
Actions may not only appear at the end, but also in the middle of a rule which is useful to perform
some processing before a rule is fully parsed. Such actions inside a rule are treated as special
nonterminals which are associated with an empty right-hand side. Thus, a rule like
x : y { action; } z
will be treated as:
x : y $act z
$act : { action; }
Actions inside a rule may also access values to the left of the action, and may return values by
assigning to the $$ value. The value returned by such an action can then be accessed by other
actions using the usual $i notation. E.g., we may write:
x : y { $$ := 2*$1; } z { $$ := $2+$3; }
which has the effect of setting the value of x to
2*(the value of y)+(the value of z).
Sometimes it is desirable to access values in enclosing rules. This can be done using the notation
$i with i ≤ 0. $0 refers to the first value “to the left” of the current rule, $-1 to the second, and
so on. Note that in this case the referenced value depends on the actual contents of the parse
stack, so you have to make sure that the requested values are always where you expect them.
There are some situations in which Delphi Yacc cannot easily determine the type of values (when
a typed parser is used). This is true, in particular, for values in enclosing rules and for the $$ value
in an action inside a rule. In such cases you may use a type cast to explicitly specify the type of
a value. The format for such type casts is $<name>$ (for left-hand side values) and $<name>i (for
right-hand side values) where name is a type identifier (which must occur in a %token, precedence
or %type definition).
11
3.7
3.7
Auxiliary Procedures
3 DELPHI YACC
Auxiliary Procedures
The third section of a Delphi Yacc program is optional. If it is present, it may contain any Delphi
code (such as supporting routines or a main program) which is tacked on to the end of the output
file.
3.8
Lexical Analysis
For any Delphi Yacc-generated parser, the programmer must supply a lexical analyzer routine
named yylex which performs the lexical analysis for the parser. This routine must be declared as
function yylex : Integer;
The yylex routine may either be prepared by hand, or by using the lexical analyzer generator
Delphi Lex (see Section Delphi Lex).
The lexical analyzer must be included in your main program behind the parser subroutine (the
yyparse code template includes a forward definition of the yylex routine such that the parser
can access the lexical analyzer). For instance, you may put the lexical analyzer routine into the
auxiliary procedures section of your Delphi Yacc grammar, either directly, or by using the the
Delphi include directive ($I).
The parser repeatedly calls the yylex routine to tokenize the input stream and obtain the individual lexical items in the input. For any literal character, the yylex routine has to return the
corresponding character code. For the other, symbolic, terminals of the input language, the lexical analyzer must return corresponding integer codes. These are assigned automatically by
Delphi Yacc in the order in which token definitions appear in the definitions section of the source
grammar. The lexical analyzer can access these values through corresponding integer constants
which are declared by Delphi Yacc in the output file.
For instance, if
%token NUM
is the first definition in the Yacc grammar, then Delphi Yacc will create a corresponding constant
declaration
const NUM = 257;
in the output file (Delphi Yacc automatically assigns symbolic token numbers starting at 257; 1
thru 255 are reserved for character literals, 0 denotes end-of-file, and 256 is reserved for the
special error token which will be discussed in Section Error Handling). This definition may then
be used, e.g., in a corresponding Delphi Lex program as follows:
[0-9]+
return(NUM);
You can also explicitly assign token numbers in the grammar. For this purpose, the first occurrence of a token identifier in the definitions section may be followed by an unsigned integer. E.g.
you may write:
12
3.9
How The Parser Works
3 DELPHI YACC
%token NUM 299
Besides the return value of yylex, the lexical analyzer routine may also return an additional
semantic value for the recognized token. This value is assigned to a variable named yylval and
may then be accessed in actions through the $i notation (see above, Section Grammar Rules and
Actions). The yylval variable is of type YYSType (the semantic value type, Integer by default);
its declaration may be found in the yyparse.cod file.
For instance, to assign an Integer value to a NUM token in the above example, we may write:
[0-9]+
begin
val(yytext, yylval, code);
return(NUM);
end;
This assigns yylval the value of the NUM token (using the Turbo Pascal standard procedure val).
If a parser uses tokens of different types (via a %token <name> definition), then the yylval variable will not be of type Integer, but instead of a corresponding variant record type which is
capable of holding all the different value types declared in the Delphi Yacc grammar. In this case,
the lexical analyzer must assign a semantic value to the corresponding record component which
is named yyname (where name stands for the corresponding type identifier).
E.g., if token NUM is declared Real:
%token <Real> NUM
then the value for token NUM must be assigned to yylval.yyReal.
3.9
How The Parser Works
Delphi Yacc uses the LALR(1) technique developed by Donald E. Knuth and F. DeRemer to
construct a simple, efficient, non-backtracking bottom-up parser for the source grammar. The
LALR parsing technique is described in detail in Aho/Sethi/Ullman (1986). It is quite instructive to
take a look at the parser description Delphi Yacc generates from a small sample grammar, to get
an idea of how the LALR parsing algorithm works. We consider the following simplified version of
the arithmetic expression grammar:
%token NUM
%left ’+’
%left ’*’
%%
expr : expr ’+’ expr
| expr ’*’ expr
| ’(’ expr ’)’
| NUM
;
When run with the -v option on the above grammar, Delphi Yacc generates the parser description
listed below.
13
3.9
How The Parser Works
3 DELPHI YACC
state 0:
$accept : _ expr $end
’(’
NUM
.
shift 2
shift 3
error
expr
goto 1
state 1:
$accept : expr _ $end
expr : expr _ ’+’ expr
expr : expr _ ’*’ expr
$end
’*’
’+’
.
accept
shift 4
shift 5
error
state 2:
expr : ’(’ _ expr ’)’
’(’
NUM
.
shift 2
shift 3
error
expr
goto 6
state 3:
expr : NUM _
.
(4)
reduce 4
state 4:
expr : expr ’*’ _ expr
’(’
NUM
.
shift 2
shift 3
error
expr
goto 7
state 5:
expr : expr ’+’ _ expr
’(’
NUM
.
shift 2
shift 3
error
14
3.9
How The Parser Works
expr
3 DELPHI YACC
goto 8
state 6:
expr : ’(’ expr _ ’)’
expr : expr _ ’+’ expr
expr : expr _ ’*’ expr
’)’
’*’
’+’
.
shift 9
shift 4
shift 5
error
state 7:
expr : expr ’*’ expr _
expr : expr _ ’+’ expr
expr : expr _ ’*’ expr
.
(2)
reduce 2
state 8:
expr : expr ’+’ expr _
expr : expr _ ’+’ expr
expr : expr _ ’*’ expr
’*’
$end
’)’
’+’
.
(1)
shift 4
reduce 1
reduce 1
reduce 1
error
state 9:
expr : ’(’ expr ’)’ _
.
(3)
reduce 3
Each state of the parser corresponds to a certain prefix of the input which has already been seen.
The parser description lists the grammar rules wich are parsed in each state, and indicates the
portion of each rule which has already been parsed by an underscore. In state 0, the start state
of the parser, the parsed rule is
$accept : expr $end
This is not an actual grammar rule, but a starting rule automatically added by Delphi Yacc. In
general, it has the format
$accept : X $end
15
3.9
How The Parser Works
3 DELPHI YACC
where X is the start nonterminal of the grammar, and $end is a pseudo token denoting end-ofinput (the $end symbol is used by the parser to determine when it has successfully parsed the
input).
The description of the start rule in state 0,
$accept : _ expr $end
with the underscore positioned before the expr symbol, indicates that we are at the beginning of
the parse and are ready to parse an expression (nonterminal expr).
The parser maintains a stack to keep track of states visited during the parse. There are two basic
kinds of actions in each state: shift, which reads an input symbol and pushes the corresponding
next state on top of the stack, and reduce which pops a number of states from the stack (corresponding to the number of right-hand side symbols of the rule used in the reduction) and consults
the goto entries of the uncovered state to find the transition corresponding to the left-hand side
symbol of the reduced rule.
In each step of the parse, the parser is in a given state (the state on top of its stack) and may
consult the current lookahead symbol, the next symbol in the input, to determine the parse action
– shift or reduce – to perform. The parser terminates as soon as it reaches state 1 and reads in
the endmarker, indicated by the accept action on $end in state 1.
Sometimes the parser may also carry out an action without inspecting the current lookahead
token. This is the case, e.g., in state 3 where the only action is reduction by rule 4:
.
reduce 4
The default action in a state can also be error indicating that any other input represents a syntax
error. (In case of such an error the parser will start syntactic error recovery, as described in
Section Error Handling.)
Now let us see how the parser responds to a given input. We consider the input string 2+5*3
which is presented to the parser as the token sequence:
NUM + NUM * NUM
Table 1 traces the corresponding actions of the parser. We also show the current state in each
move, and the remaining states on the stack.
It is also instructive to see how the parser responds to illegal inputs. E.g., you may try to figure
out what the parser does when confronted with:
NUM + )
or:
( NUM * NUM
You will find that the parser, sooner or later, will always run into an error action when confronted
with errorneous inputs. An LALR parser will never shift an invalid symbol and thus will always find
syntax errors as soon as it is possible during a left-to-right scan of the input.
Delphi Yacc provides a debugging option (-d) that may be used to trace the actions performed by
the parser. When a grammar is compiled with the -d option, the generated parser will print out
the actions as it parses its input.
16
3.10
Ambigious Grammars
S TACK
3 DELPHI YACC
S TATE
0
3
L OOKAHEAD
NUM
0
1
5
3
0
10
510
+
NUM
8
4
3
510
8510
48510
*
NUM
7
48510
8
510
$end
1
0
$end
ACTION
shift state 3
reduce rule 4 (pop 1 state, uncovering state 0, then
goto state 1 on symbol expr)
shift state 5
shift state 3
reduce rule 4 (pop 1 state, uncovering state 5, then
goto state 8 on symbol expr)
shift 4
shift 3
reduce rule 4 (pop 1 state, uncovering state 4, then
goto state 7 on symbol expr)
reduce rule 2 (pop 3 states, uncovering state 5, then
goto state 8 on symbol expr)
reduce rule 1 (pop 3 states, uncovering state 0, then
goto state 1 on symbol expr)
accept
Table 1: Parse of "NUM + NUM * NUM".
3.10
Ambigious Grammars
There are situations in which Delphi Yacc will not produce a valid parser for a given input language. LALR(1) parsers are restricted to one-symbol lookahead on which they have to base
their parsing decisions. If a grammar is ambigious, or cannot be parsed unambigiously using
one-symbol lookahead, Delphi Yacc will generate parsing conflicts when constructing the parse
table. There are two types of such conflicts: shift/reduce conflicts (when there is both a shift and
a reduce action for a given input symbol in a given state), and reduce/reduce conflicts (if there is
more than one reduce action for a given input symbol in a given state). Note that there never will
be a shift/shift conflict.
When a grammar generates parsing conflicts, Delphi Yacc prints out the number of shift/reduce
and reduce/reduce conflicts it encountered when constructing the parse table. However, Delphi
Yacc will still generate the output code for the parser. To resolve parsing conflicts, Delphi Yacc
uses the following built-in disambiguating rules:
• in a shift/reduce conflict, Delphi Yacc chooses the shift action.
• in a reduce/reduce conflict, Delphi Yacc chooses reduction of the first grammar rule.
The shift/reduce disambiguating rule correctly resolves a type of ambiguity known as the “danglingelse ambiguity” which arises in the syntax of conditional statements of many programming languages (as in Pascal):
%token IF THEN ELSE
%%
stmt : IF expr THEN stmt
| IF expr THEN stmt ELSE stmt
;
This grammar is ambigious, because a nested construct like
17
3.10
Ambigious Grammars
3 DELPHI YACC
IF expr-1 THEN IF expr-2 THEN stmt-1
ELSE stmt-2
can be parsed two ways, either as:
IF expr-1 THEN ( IF expr-2 THEN stmt-1
ELSE stmt-2 )
or as:
IF expr-1 THEN ( IF expr-2 THEN stmt-1 )
ELSE stmt-2
The first interpretation makes an ELSE belong to the last unmatched IF which also is the interpretation chosen in most programming languages. This is also the way that a Delphi Yacc-generated
parser will parse the construct since the shift/reduce disambiguating rule has the effect of neglecting the reduction of IF expr-2 THEN stmt-1; instead, the parser will shift the ELSE symbol
which eventually leads to the reduction of IF expr-2 THEN stmt-1 ELSE stmt-2.
The reduce/reduce disambiguating rule is used to resolve conflicts that arise when there is more
than one grammar rule matching a given construct. Such ambiguities are often caused by “special
case constructs” which may be given priority by simply listing the more specific rules ahead of
the more general ones.
For instance, the following is an excerpt from the grammar describing the input language of the
UNIX equation formatter EQN:
%right
%%
expr :
|
|
;
SUB SUP
expr SUB expr SUP expr
expr SUB expr
expr SUP expr
Here, the SUB and SUP operator symbols denote sub- and superscript, respectively. The rationale behind this example is that an expression involving both sub- and superscript is often set
differently from a superscripted subscripted expression (compare xni to xi n ). This special case
is therefore caught by the first rule in the above example which causes a reduce/reduce conflict
with rule 3 in expressions like expr-1 SUB expr-2 SUP expr-3. The conflict is resolved in favour
of the first rule.
In both cases discussed above, the ambiguities could also be eliminated by rewriting the grammar
accordingly (although this yields more complicated and less readable grammars). This may not
always be the case. Often ambiguities are also caused by design errors in the grammar. Hence,
if Delphi Yacc reports any parsing conflicts when constructing the parser, you should use the -v
option to generate the parser description (.lst file) and check whether Delphi Yacc resolved the
conflicts correctly.
There is one type of syntactic constructs for which one often deliberately uses an ambigious
grammar as a more concise representation for a language that could also be specified unambigiously: the syntax of expressions. For instance, the following is an unambigious grammar for
simple arithmetic expressions:
18
3.10
Ambigious Grammars
3 DELPHI YACC
%token NUM
%%
expr
: term
| expr ’+’ term
;
term
: factor
| term ’*’ factor
;
factor
: ’(’ expr ’)’
| NUM
;
You may check yourself that this grammar gives * a higher precedence than + and makes both operators left-associative. The same effect can be achieved with the following ambigious grammar
using precedence definitions:
%token NUM
%left ’+’
%left ’*’
%%
expr : expr ’+’ expr
| expr ’*’ expr
| ’(’ expr ’)’
| NUM
;
Without the precedence definitions, this is an ambigious grammar causing a number of shift/reduce
conflicts. The precedence definitions are used to correctly resolve these conflicts (conflicts resolved using precedence will not be reported by Delphi Yacc).
Each precedence definition introduces a new precedence level (lowest precedence first) and
specifies whether the corresponding operators should be left-, right- or nonassociative (nonassociative operators cannot be combined at all; example: relational operators in Pascal).
Delphi Yacc uses precedence information to resolve shift/reduce conflicts as follows. Precedences are associated with each terminal occuring in a precedence definition. Furthermore,
each grammar rule is given the precedence of its rightmost terminal (this default choice can be
overwritten using a %prec tag; see below). To resolve a shift/reduce conflict using precedence,
both the symbol and the rule involved must have been assigned precedences. Delphi Yacc then
chooses the parse action as follows:
• If the symbol has higher precedence than the rule: shift.
• If the rule has higher precedence than the symbol: reduce.
• If symbol and rule have the same precedence, the associativity of the symbol determines
the parse action: if the symbol is left-associative: reduce; if the symbol is right-associative:
shift; if the symbol is non-associative: error.
19
3.10
Ambigious Grammars
3 DELPHI YACC
To give you an idea of how this works, let us consider our ambigious arithmetic expression grammar (without precedences):
%token
%%
expr :
|
|
|
;
NUM
expr ’+’ expr
expr ’*’ expr
’(’ expr ’)’
NUM
This grammar generates four shift/reduce conflicts. The description of state 8 reads as follows:
state 8:
*** conflicts:
shift 4, reduce 1 on ’*’
shift 5, reduce 1 on ’+’
expr : expr ’+’ expr _
expr : expr _ ’+’ expr
expr : expr _ ’*’ expr
’*’
’+’
$end
’)’
.
(1)
shift 4
shift 5
reduce 1
reduce 1
error
In this state, we have successfully parsed a + expression (rule 1). When the next symbol is + or *,
we have the choice between the reduction and shifting the symbol. Using the default shift/reduce
disambiguating rule, Delphi Yacc has resolved these conflicts in favour of shift.
Now let us assume the above precedence definition:
%left ’+’
%left ’*’
which gives * higher precedence than + and makes both operators left-associative. The rightmost
terminal in rule 1 is +. Hence, given these precedence definitions, the first conflict will be resolved
in favour of shift (* has higher precedence than +), while the second one is resolved in favour of
reduce (+ is left-associative).
Similar conflicts arise in state 7:
state 7:
*** conflicts:
shift 4, reduce 2 on ’*’
20
3.10
Ambigious Grammars
3 DELPHI YACC
shift 5, reduce 2 on ’+’
expr : expr ’*’ expr _
expr : expr _ ’+’ expr
expr : expr _ ’*’ expr
’*’
’+’
$end
’)’
.
(2)
shift 4
shift 5
reduce 2
reduce 2
error
Here, we have successfully parsed a * expression which may be followed by another + or *
operator. Since * is left-associative and has higher precedence than +, both conflicts will be
resolved in favour of reduce.
Of course, you can also have different operators on the same precedence level. For instance,
consider the following extended version of the arithmetic expression grammar:
%token NUM
%left ’+’ ’-’
%left ’*’ ’/’
%%
expr
: expr ’+’
| expr ’-’
| expr ’*’
| expr ’/’
| ’(’ expr
| NUM
;
expr
expr
expr
expr
’)’
This puts all “addition” operators on the first and all “multiplication” operators on the second
precedence level. All operators are left-associative; for instance, 5+3-2 will be parsed as (5+3)-2.
By default, Delphi Yacc assigns each rule the precedence of its rightmost terminal. This is a
sensible decision in most cases. Occasionally, it may be necessary to overwrite this default
choice and explicitly assign a precedence to a rule. This can be done by putting a precedence
tag of the form
%prec symbol
at the end of the corresponding rule which gives the rule the precedence of the specified symbol.
For instance, to extend the expression grammar with a unary minus operator, giving it highest
precedence, you may write:
%token NUM
%left ’+’ ’-’
%left ’*’ ’/’
%right UMINUS
%%
expr
: expr ’+’ expr
| expr ’-’ expr
21
3.11
Error Handling
|
|
|
|
|
;
3 DELPHI YACC
expr ’*’ expr
expr ’/’ expr
’-’ expr
%prec UMINUS
’(’ expr ’)’
NUM
Note the use of the UMINUS token which is not an actual input symbol but whose sole purpose it
is to give unary minus its proper precedence. If we omitted the precedence tag, both unary and
binary minus would have the same precedence because they are represented by the same input
symbol.
3.11
Error Handling
Syntactic error handling is a difficult area in the design of user-friendly parsers. Usually, you
will not like to have the parser give up upon the first occurrence of an errorneous input symbol.
Instead, the parser should recover from a syntax error, that is, it should try to find a place in the
input where it can resume the parse.
Delphi Yacc provides a general mechanism to implement parsers with error recovery. A special
predefined error token may be used in grammar rules to indicate positions where syntax errors
might occur. When the parser runs into an error action (i.e., reads an errorneous input symbol) it
prints out an error message and starts error recovery by popping its stack until it uncovers a state
in which there is a shift action on the error token. If there is no such state, the parser terminates
with return value 1, indicating an unrecoverable syntax error. If there is such a state, the parser
takes the shift on the error token (pretending it has seen an imaginary error token in the input),
and resumes parsing in a special “error mode.”
While in error mode, the parser quietly skips symbols until it can again perform a legal shift action.
To prevent a cascade of error messages, the parser returns to its normal mode of operation only
after it has seen and shifted three legal input symbols. Any additional error found after the first
shifted symbol restarts error recovery, but no error message is printed. The Delphi Yacc library
routine yyerrok may be used to reset the parser to its normal mode of operation explicitly.
For a simple example, consider the rule
stmt : error ’;’ { yyerrok; }
and assume a syntax error occurs while a statement (nonterminal stmt) is parsed. The parser
prints an error message, then pops its stack until it can shift the token error of the error rule.
Proceeding in error mode, it will skip symbols until it finds a semicolon, then reduces by the error
rule. The call to yyerrok tells the parser that we have recovered from the error and that it should
proceed with the normal parse. This kind of “panic mode” error recovery scheme works well when
statements are always terminated with a semicolon. The parser simply skips the “bad” statement
and then resumes the parse.
Implementing a good error recovery scheme can be a difficult task; see Aho/Sethi/Ullman (1986)
for a more comprehensive treatment of this topic. Schreiner and Friedman have developed a
systematic technique to implement error recovery with Yacc which I found quite useful (I used it
myself to implement error recovery in the Delphi Yacc parser); see Schreiner/Friedman (1985).
22
3.12
3.12
Yacc Library
3 DELPHI YACC
Yacc Library
The Delphi Yacc library (YaccLib) unit provides some global declarations used by the parser
routine yyparse, and some variables and utility routines which may be used to control the actions
of the parser and to implement error recovery. See the file yacclib.pas for a description of these
variables and routines.
You can also modify the Yacc library unit (and/or the code template in the yyparse.cod file) to
customize Delphi Yacc to your target applications.
3.13
Other Features
Delphi Yacc supports all additional language elements entitled as “Old Features Supported But
not Encouraged” in the UNIX manual, which are provided for backward compatibility with older
versions of (UNIX) Yacc:
• literals delimited by double quotes.
• multiple-character literals. Note that these are not treated as character sequences but represent single tokens which are given a symbolic integer code just like any other token identifier. However, they will not be declared in the output file, so you have to make sure yourself
that the lexical analyzer returns the correct codes for these symbols. E.g., you might explicitly assign token numbers by using a definition like
%token ’:=’ 257
at the beginning of the Yacc grammar.
• \ may be used instead of %, i.e. \\ means %%, \left is the same as %left, etc.
• other synonyms:
– %< for %left
– %> for %right
– %binary or %2 for %nonassoc
– %term or %0 for %token
– %= for %prec
• actions may also be written as = { ... } or = single-statement;
• Delphi declarations (%{ ... %}) may be put at the beginning of the rules section. They will
be treated as local declarations of the actions routine.
3.14
Implementation Restrictions
As with Delphi Lex, internal table sizes and the main memory available limit the complexity of
source grammars that Delphi Yacc can handle. However, the maximum table sizes provided by
Delphi Yacc are large enough to handle quite complex grammars (such as the Pascal grammar in
the Delphi Yacc distribution). The actual table sizes are shown in the statistics printed by Delphi
Yacc when a compilation is finished. The given figures are ”s” (states), ”i” (LR0 kernel items), ”t”
(shift and goto transitions) and ”r” (reductions).
23
3.15
Differences from UNIX Yacc
3 DELPHI YACC
The default stack size of the generated parsers is yymaxdepth = 1024, as declared in the Delphi
Yacc library unit. This should be sufficient for any average application, but you can change the
stack size by including a corresponding declaration in the definitions part of the Yacc grammar
(or change the value in the YaccLib unit). Note that right-recursive grammar rules may increase
stack space requirements, so it is a good idea to use left-recursive rules wherever possible.
3.15
Differences from UNIX Yacc
Major differences between Delphi Yacc and UNIX Yacc are listed below.
• Delphi Yacc produces output code for Delphi, rather than for C.
• Delphi Yacc does not support %union definitions. Instead, a value type is declared by specifying the type identifier itself as the tag of a %token or %type definition. Delphi Yacc will
automatically generate an appropriate variant record type (YYSType) which is capable of
holding values of any of the types used in %token and %type.
Type checking is very strict. If you use type definitions, then any symbol referred to in an
action must have a type introduced in a type definition. Either the symbol must have been
assigned a type in the definitions section, or the $<type-identifier> notation must be
used. The syntax of the %type definition has been changed slightly to allow definitions of
the form
%type <type-identifier>
(omitting the nonterminals) which may be used to declare types which are not assigned to
any grammar symbol, but are used with the $<...> construct.
• The parse tables constructed by this Yacc version are slightly greater than those constructed
by UNIX Yacc, since a reduce action will only be chosen as the default action if it is the only
action in the state. In difference, UNIX Yacc chooses a reduce action as the default action
whenever it is the only reduce action of the state (even if there are other shift actions).
This solves a bug in UNIX Yacc that makes the generated parser start error recovery too
late with certain types of error productions (see also Schreiner/Friedman, Introduction to
compiler construction with UNIX, 1985). Also, errors will be caught sooner in most cases
where UNIX Yacc would carry out an additional (default) reduction before detecting the
error.
• Library routines are named differently from the UNIX version (e.g., the yyerrlab routine
takes the place of the YYERROR macro of UNIX Yacc), and, of course, all macros of UNIX
Yacc (YYERROR, YYACCEPT, etc.) had to be implemented as procedures.
24
4
4
DELPHI LEX
Delphi Lex
This section describes the Delphi Lex lexical analyzer generator.
4.1
Usage
dlex [options] lex-file[.l] [output-file[.pas]]
4.2
Options

“Verbose:” Lex generates a readable description of the generated lexical analyzer, written
”-v”
to lex-file with new extension .lst.
"-o" “Optimize:” Lex optimizes DFA tables to produce a minimal DFA.
4.3
Description
Delphi Lex is a program generator that is used to generate the Delphi source code for a lexical
analyzer subroutine from the specification of an input language by a regular expression grammar.
Delphi Lex parses the source grammar contained in lex-file (with default suffix .l) and writes
the constructed lexical analyzer subroutine to the specified output-file (with default suffix .pas);
if no output file is specified, output goes to lex-file with new suffix .pas. If any errors are found
during compilation, error messages are written to the list file (lex-file with new suffix .lst).
The generated output file contains a lexical analyzer routine, yylex, implemented as:
function yylex : Integer;
This routine has to be called by your main program to execute the lexical analyzer. The return
value of the yylex routine usually denotes the number of a token recognized by the lexical analyzer (see the return routine in the LexLib unit). At end-of-file the yylex routine normally returns
0.
The code template for the yylex routine may be found in the yylex.cod file. This file is needed
by Delphi Lex when it constructs the output file. It must be present either in the current directory
or in the directory from which Delphi Lex was executed (Delphi Lex searches these directories in
the indicated order). (NB: For the Linux/Free Pascal version, the code template is searched in
some directory defined at compile-time instead of the execution path, usually /usr/lib/fpc/lexyacc.)
The Delphi Lex library (LexLib) unit is required by programs using Lex-generated lexical analyzers; you will therefore have to put an appropriate uses clause into your program or unit that
contains the lexical analyzer routine. The LexLib unit also provides various useful utility routines;
see the file lexlib.pas for further information.
25
4.4
Lex Source
4.4
4
DELPHI LEX
Lex Source
A Delphi Lex program consists of three sections separated with the %% delimiter:
definitions
%%
rules
%%
auxiliary procedures
All sections may be empty. The Delphi Lex language is line-oriented; definitions and rules are
separated by line breaks. There is no special notation for comments, but (Delphi style) comments
may be included as Delphi fragments (see below).
The definitions section may contain the following elements:
• regular definitions in the format:
name
substitution
which serve to abbreviate common subexpressions. The {name} notation causes the corresponding substitution from the definitions section to be inserted into a regular expression.
The name must be a legal identifier (letter followed by a sequence of letters and digits; the
underscore counts as a letter; upper- and lowercase are distinct). Regular definitions must
be non-recursive.
• start state definitions in the format:
%start name ...
which are used in specifying start conditions on rules (described below). The %start keyword may also be abbreviated as %s or %S.
• Delphi declarations enclosed between %{ and %}. These will be inserted into the output file
(at global scope). Also, any line that does not look like a Lex definition (e.g., starts with
blank or tab) will be treated as Delphi code. (In particular, this also allows you to include
Delphi comments in your Lex program.)
The rules section of a Delphi Lex program contains the actual specification of the lexical analyzer
routine. It may be thought of as a big CASE statement discriminating over the different patterns to
be matched and listing the corresponding statements (actions) to be executed. Each rule consists
of a regular expression describing the strings to be matched in the input, and a corresponding
action, a Delphi statement to be executed when the expression matches. Expression and statement are delimited with whitespace (blanks and/or tabs). Thus the format of a Lex grammar rule
is:
expression
statement;
Note that the action must be a single Delphi statement terminated with a semicolon (use begin ... end
for compound statements). The statement may span multiple lines if the successor lines are indented with at least one blank or tab. The action may also be replaced by the | character,
indicating that the action for this rule is the same as that for the next one.
26
4.5
Regular Expressions
4
E XPRESSION
c
\c
"s"
.
^
$
[s]
[^s]
r*
r+
r?
r{m,n}
r{m}
r1 r2
r1 |r2
(r)
r1 /r2
<x>r
M ATCHES
any non-operator character c
character c literally
string s literally
any character but newline
beginning of line
end of line
any character in s
any character not in s
zero or more r’s
one or more r’s
zero or one r
m to n occurrences of r
m occurrences of r
r1 then r2
r1 or r2
r
r1 when followed by r2
r when in start condition x
DELPHI LEX
E XAMPLE
a
\*
"**"
a.*b
^abc
abc$
[abc]
[^abc]
a*
a+
a?
a{1,5}
a{5}
ab
a|b
(a|b)
a/b
<x>abc
Table 2: Regular expressions.
The Delphi Lex library unit provides various variables and routines which are useful in the programming of actions. In particular, the yytext string variable holds the text of the matched string,
and the yyleng Byte variable its length.
Regular expressions are used to describe the strings to be matched in a grammar rule. They
are built from the usual constructs describing character classes and sequences, and operators
specifying repetitions and alternatives. The precise format of regular expressions is described in
the next section.
The rules section may also start with some Delphi declarations (enclosed in %{ %}) which are
treated as local declarations of the actions routine.
Finally, the auxiliary procedures section may contain arbitrary Turbo Pascal code (such as supporting routines or a main program) which is simply tacked on to the end of the output file. The
auxiliary procedures section is optional.
4.5
Regular Expressions
Table 2 summarizes the format of the regular expressions recognized by Delphi Lex (also compare Aho, Sethi, Ullman 1986, fig. 3.48). c stands for a single character, s for a string, r for a
regular expression, and n, m for nonnegative integers.
The operators *, +, ? and {} have highest precedence, followed by concatenation. The | operator
has lowest precedence. Parentheses () may be used to group expressions and overwrite default
precedences. The <> and / operators may only occur once in an expression.
The usual C-like escapes are recognized:
• \n denotes newline
• \r denotes carriage return
27
4.6
Start Conditions
4
DELPHI LEX
• \t denotes tab
• \b denotes backspace
• \f denotes form feed
• \nnn denotes character no. nnn in octal base
You can also use the \ character to quote characters which would otherwise be interpreted as operator symbols. In character classes, you may use the - character to denote ranges of characters.
For instance, [a-z] denotes the class of all lowercase letters.
The expressions in a Delphi Lex program may be ambigious, i.e. there may be inputs which
match more than one rule. In such a case, the lexical analyzer prefers the longest match and,
if it still has the choice between different rules, it picks the first of these. If no rule matches,
the lexical analyzer executes a default action which consists of copying the input character to
the output unchanged. Thus, if the purpose of a lexical analyzer is to translate some parts of
the input, and leave the rest unchanged, you only have to specify the patterns which have to be
treated specially. If, however, the lexical analyzer has to absorb its whole input, you will have to
provide rules that match everything. E.g., you might use the rules
.
\n
|
;
which match “any other character” (and ignore it).
Sometimes certain patterns have to be analyzed differently depending on some amount of context
in which the pattern appears. In such a case the / operator is useful. For instance, the expression
a/b matches a, but only if followed by b. Note that the b does not belong to the match; rather, the
lexical analyzer, when matching an a, will look ahead in the input to see whether it is followed by
a b, before it declares that it has matched an a. Such lookahead may be arbitrarily complex (up
to the size of the LexLib input buffer). E.g., the pattern a/.*b matches an a which is followed by
a b somewhere on the same input line. Delphi Lex also has a means to specify left context which
is described in the next section.
4.6
Start Conditions
Delphi Lex provides some features which make it possible to handle left context. The ^ character
at the beginning of a regular expression may be used to denote the beginning of the line. More
distant left context can be described conveniently by using start conditions on rules.
Any rule which is prefixed with the <> construct is only valid if the lexical analyzer is in the denoted
start state. For instance, the expression <x>a can only be matched if the lexical analyzer is in
start state x. You can have multiple start states in a rule; e.g., <x,y>a can be matched in start
states x or y.
Start states have to be declared in the definitions section by means of one or more start state
definitions (see above). The lexical analyzer enters a start state through a call to the LexLib
routine start. E.g., you may write:
%start x y
%%
<x>a
start(y);
28
4.7
Lex Library
4
DELPHI LEX
<y>b
start(x);
%%
begin
start(x); if yylex=0 then ;
end.
Upon initialization, the lexical analyzer is put into state x. It then proceeds in state x until it
matches an a which puts it into state y. In state y it may match a b which puts it into state x again,
etc.
Start conditions are useful when certain constructs have to be analyzed differently depending on
some left context (such as a special character at the beginning of the line), and if multiple lexical
analyzers have to work in concert. If a rule is not prefixed with a start condition, it is valid in all
user-defined start states, as well as in the lexical analyzer’s default start state.
4.7
Lex Library
The Delphi Lex library (LexLib) unit provides various variables and routines which are used
by Lex-generated lexical analyzers and application programs. It provides the input and output
streams and other internal data structures used by the lexical analyzer routine, and supplies
some variables and utility routines which may be used by actions and application programs. Refer to the file lexlib.pas for a closer description.
You can also modify the Lex library unit (and/or the code template in the yylex.cod file) to customize Delphi Lex to your target applications. E.g., you might wish to optimize the code of the
lexical analyzer for some special application, make the analyzer read from/write to memory instead of files, etc.
4.8
Implementation Restrictions
Internal table sizes and the main memory available limit the complexity of source grammars that
Delphi Lex can handle. There is currently no possibility to change internal table sizes (apart from
modifying the sources of Delphi Lex itself), but the maximum table sizes provided by Delphi Lex
seem to be large enough to handle most realistic applications. The actual table sizes depend on
the particular implementation (they are much larger than the defaults if TP Lex has been compiled
with one of the 32 bit compilers such as Delphi 2 or Free Pascal), and are shown in the statistics
printed by Delphi Lex when a compilation is finished. The units given there are “p” (positions, i.e.
items in the position table used to construct the DFA), “s” (DFA states) and “t” (transitions of the
generated DFA).
As implemented, the generated DFA table is stored as a typed array constant which is inserted
into the yylex.cod code template. The transitions in each state are stored in order. Of course
it would have been more efficient to generate a big CASE statement instead, but I found that this
may cause problems with the encoding of large DFA tables because Delphi has a quite rigid limit
on the code size of individual procedures. I decided to use a scheme in which transitions on
different symbols to the same state are merged into one single transition (specifying a character
set and the corresponding next state). This keeps the number of transitions in each state quite
small and still allows a fairly efficient access to the transition table.
The Delphi Lex program has an option (-o) to optimize DFA tables. This causes a minimal DFA to
be generated, using the algorithm described in Aho, Sethi, Ullman (1986). Although the absolute
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4.9
Differences from UNIX Lex
4
DELPHI LEX
limit on the number of DFA states that Delphi Lex can handle is at least 300, Delphi Lex poses an
additional restriction (100) on the number of states in the initial partition of the DFA optimization
algorithm. Thus, you may get a fatal integer set overflow message when using the -o option
even when Delphi Lex is able to generate an unoptimized DFA. In such cases you will just have
to be content with the unoptimized DFA. (Hopefully, this will be fixed in a future version. Anyhow,
using the merged transitions scheme described above, Delphi Lex usually constructs unoptimized
DFA’s which are not far from being optimal, and thus in most cases DFA optimization won’t have
a great impact on DFA table sizes.)
4.9
Differences from UNIX Lex
Major differences between Delphi Lex and UNIX Lex are listed below.
• Delphi Lex produces output code for Delphi, rather than for C.
• Character tables (%T) are not supported; neither are any directives to determine internal
table sizes (%p, %n, etc.).
• Library routines are named differently from the UNIX version (e.g., the start routine takes
the place of the BEGIN macro of UNIX Lex), and, of course, all macros of UNIX Lex (ECHO,
REJECT, etc.) had to be implemented as procedures.
• The Delphi Lex library unit starts counting line numbers at 0, incrementing the count before
a line is read (in contrast, UNIX Lex initializes yylineno to 1 and increments it after the line
end has been read). This is motivated by the way in which Delphi Lex maintains the current
line, and will not affect your programs unless you explicitly reset the yylineno value (e.g.,
when opening a new input file). In such a case you should set yylineno to 0 rather than 1.
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